Goninan in Part One: The Hidden People, "Border Spirit" p. 336
The Little Country (1991)
“A weapon you held and didn't know how to use belonged to your enemy.”
Source: Making Money
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Terry Pratchett 796
English author 1948–2015Related quotes

1960s

“Your virtuous living is your enemy's best and cheapest weapon”
Sketchbook 1946-1949

“To know your Enemy, you must become your Enemy.”
This is sometimes attributed to Sun Tzu in combination with the above quote, as well as alone, but it too has not been sourced to any published translation of The Art of War, though it is similar in concept to his famous statement in Ch. 3 : "It is said that if you know your enemies and know yourself, you will not be imperiled in a hundred battles..."
Misattributed

“A weapon is a device for making your enemy change his mind.”
Source: Vorkosigan Saga, The Vor Game (1990)

"My Enemy"
In Exile Deo (2004)